1. Field of the Invention
The present invention relates to diesel cycle engines and more particularly to diesel cycle engines having auxiliary combustion chambers or so-called swirl chambers. More specifically, the present invention pertains to exhaust gas recirculation means for such swirl chamber type diesel engines.
2. Description of the Prior Art
In the art of diesel cycle engines, the exhaust gas from the engine includes noxious components such as nitrogen oxides, carbon monoxide and so-called particulates, which include smoke forming carbon particles, particles of liquid fuel containing hydrocarbon and other particles such as metal particles. In order to decrease the emission of nitrogen oxides, an exhaust gas recirculation system has been adopted to return a part of the exhaust gas to the intake system in an amount which is controlled in accordance with the engine operating condition. In view of the fact that the excess air which is not used for combustion in diesel engines decreases in response to an increase in the engine load, it has been a common practice to decrease the amount of the recirculation gas or to stop the recirculation of the exhaust gas. For example, Japanese patent publication No. 54-17092 describes a control wherein the exhaust gas recirculation ratio, which is defined as a ratio of the amount of the recirculated gas to the amount of the total intake gas, is decreased in accordance with an increase in the engine load. U.S. Pat. No. 4,369,753 teaches to increase the exhaust gas recirculation under light load operation.
In diesel engines of the swirl chamber type, it has already been proposed to provide the swirl chamber with a heat insulating structure in order to maintain a satisfactorily high temperature in the swirl chamber even under light load or low temperature engine operation. For example, U.S. Pat. No. 3,259,116 discloses a diesel engine having an auxiliary or swirl chamber made of a refractory material located in the cylinder head with a heat insulative space encircling the auxiliary chamber. The Japanese utility model application No. 57-71421 filed on May 18, 1982 and disclosed for public inspection under the disclosure No. 58-175118 on Nov. 22, 1983 teaches to provide the auxiliary chamber with a refractory ceramic material and locate the chamber in the cylinder head with a heat insulative space therebetween. The auxiliary chamber may partly or wholly be made of a heat insulative ceramic material. For example, the upper half of the auxiliary chamber may be made of a heat insulative ceramic material such as zirconum oxides, and the lower half, which is adapted to be located adjacent to the top surface of the cylinder, may be made of a refractory ceramic material such as silicon nitrides, which have a high thermal shock resistance. This structure may make the heat insulative space unnecessary.
In the diesel engines having auxiliary chambers of a heat insulative structure, it is possible to increase temperature in the chamber so that the combustion of fuel can be enhanced. As the result, it is possible to obtain an increased engine output as compared with a conventional diesel engine having an auxiliary chamber, of which an upper half is made of aluminum alloy and a lower half made of steel or cast iron. It should also be pointed out that the increase in temperature of the chamber is effective to decrease the emission of the particulates in the exhaust gas. It has however been recognized that, in this type of engine, the increased combustion temperature causes an increase in the emission of nitrogen oxides and that conventional controls of exhaust gas recirculation cannot effectively suppress the emission of nitrogen oxides and sometimes have adverse effects on the emission of particulates.